Add aliases to Kamal

Aliases are defined in the configuration file under the `aliases` key.

The configuration is a map of alias name to command. When we run the
command the we just do a literal replacement of the alias with the
string.

So if we have:

```yaml
aliases:
  console: app exec -r console -i --reuse "rails console"
```

Then running `kamal console -r workers` will run the command

```sh
$ kamal app exec -r console -i --reuse "rails console" -r workers
```

Because of the order Thor parses the arguments, this allows us to
override the role from the alias command.

There might be cases where we need to munge the command a bit more but
that would involve getting into Thor command parsing internals,
which are complicated and possibly subject to change.

There's a chance that your aliases could conflict with future built-in
commands, but there's not likely to be many of those and if it happens
you'll get a validation error when you upgrade.

Thanks to @dhnaranjo for the idea!

test/fixtures/deploy_with_aliases.yml

@@ -0,0 +1,20 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    - 1.1.1.1
+    - 1.1.1.2
+  workers:
+    hosts:
+      - 1.1.1.3
+      - 1.1.1.4
+  console:
+    hosts:
+      - 1.1.1.5
+registry:
+  username: user
+  password: pw
+aliases:
+  info: details
+  console: app exec --reuse -p -r console "bin/console"
+  exec: app exec --reuse -p -r console